"Raising serum testosterone to normal reduced body weight, waist circumference and blood pressure, and improved metabolic profiles," said a statement on the study led by Farid Saad of German pharmaceutical giant Bayer Pharma.
The improvements were progressive over the five years of the study.
"Increased testosterone levels improve energy and motivation to do physical exercise and more movement in general; testosterone also increases lean body mass (fat free mass), increasing the energy used by patients," the statement said.
Approached for comment, Sanjay Kinra, an obesity expert and researcher at London School of Hygiene and Tropical Medicine, urged caution.
"It is quite possible that a drug that is improving the mood of middle-aged people over a period of time will likely make them a bit more active and help them lose a little bit of weight, but it is a serious drug, testosterone, and it causes serious health effects," he told AFP.
Testosterone, a steroid hormone secreted by the male testes and to a lesser extent by the female ovaries that affects brain development and sexual behavior, has been linked by some researchers to prostate cancer and heart disease.
Kinra said there were "hundreds of things out there" that could make people lose weight in the short term, but only a change of lifestyle would ensure a long-term thinner self.
"As a mass treatment of obesity it (testosterone) is not meaningful because you are not going to trade off your risk of getting prostate cancer or heart disease for a bit of weight loss that is not going to be lifelong anyway, it will only remain for the time that you are on testosterone."